If Caltrops are activated their effect last for 6 seconds, matching the skill description. But the skill description doesn't mention how long can the trap remain non-activated on the ground, and whether there's anything else which can make Caltrops disappear. What are those details?
Caltrops remain on the ground for 30 seconds if they are not activated.
Also, only five caltrops can be active at a time—laying a sixth caltrop makes the oldest one disappear.
In addition, zone transitions (e.g., entering a dungeon or descending a dungeon floor) immediately remove all caltrops from the previous zone.